GLWidget Description

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GLWidget Description

• Jason Goffeney
• 3/8/2006

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GLWidget

• The GLWidget class extends the Qt QGLWidget.
• The QGLWidget is a Qt Widget that happens to have
  an OpenGL drawing context inside of it which
  responds to OpenGL commands.
• By extending the QGLWidget it allows us to
  override the original methods and insert our own
  code for drawing, handling user input, etc.

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GLWidget

• When a QGLWidget (or an object that extends it)
  is created certain methods are immediately called
  in the following order
• Everything in the constructor is completed
• The method initializeGL is called to set any
  variables or properties used by OpenGL
• The method resizeGL is called to set up the GL
  window
• The method paintGL is called to begin the drawing

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GLWidget Constructor

• Most of what happens in the constructor is just
  initializing variables and creating the dialog
  boxes for the application (inputs for the initial
  shapes and the print dialog).
• The interesting bits
• The Timer
• Mouse Tracking

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GLWidget Constructor Timer

• The timer is used to regulate the maximum number
  of times per second the graphics will be drawn to
  the screen.
• QTimer timer new QTimer(this)
• connect(timer, SIGNAL(timeout()), this,
  SLOT(updateGL()))
• timer-gtstart(20)

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GLWidget Constructor Timer

• Create
• Takes the parent widget as its parameter
• QTimer timer new QTimer(this)
• Connect
• Every time the timer hits its limit it emits the
  signal timeout which is connected to the
  QGLWidget slot updateGL which itself calls the
  paintGL method.
• connect(timer, SIGNAL(timeout()), this,
  SLOT(updateGL()))
• Start
• Starts the timer counting and it takes the number
  in milliseconds it will count to before emitting
  timeout and starting its count over.
• timer-gtstart(20)

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GLWidget Constructor Mouse Tracking

• Mouse tracking used to indicate whether we want
  to passively track the mouse cursor in the widget
  without any buttons needing to be pressed.
• The would be standard for any game or simulation
  where looking around the scene is performed via
  moving the mouse.
• setMouseTracking(true)

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GLWidget initializeGL

• This method is used to setup any variables or
  properties necessary before any drawing is done.
• There are easily dozens of different properties
  that can be set up before drawing including
• Light Sources
• Clearing Color
• How objects are shaded
• etc.

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GLWidget resizeGL

• This method is called after initializeGL and
  anytime the widget is resized.
• It basically is responsible for setting up the
  view in terms of its dimensions, where the
  viewpoint is located, and the method of
  displaying depth information (2D orthographic, 3D
  orthographic or 3D perspective).

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GLWidget resizeGL

• This method is called after initializeGL and
  anytime the widget is resized.
• It basically is responsible for setting up the
  view in terms of its dimensions, where the
  viewpoint is located, and the method of
  displaying depth information (2D orthographic, 3D
  orthographic or 3D perspective).

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GLWidget resizeGL

• The method takes height and width as its
  parameters but is never explicitly called.
• Anytime the widget is resized this method is
  called and its new height and width are
  automatically passed as the parameters.

w
h
w
h
resizeGL(int w, int h)
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GLWidget resizeGL

• The first command to issue in resizeGL should be
• glMatrixMode(GL_PROJECTION)
• glLoadIdentity()
• OpenGL operates on two different stacks of
  matrices for doing various transformation.
• GL_PROJECTION contains a matrix for projection
  transformations which describe the viewing volume
• GL_MODELVIEW contains a matrix for modeling
  transforming that affect the scene

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GLWidget resizeGL

• The first command to issue in resizeGL should be
• glMatrixMode(GL_PROJECTION)
• glLoadIdentity()
• By setting the matrix mode to GL_PROJECTION it
  specifies which matrix stack to operate on.
• The call of glLoadIdentity then sets the current
  projection matrix to the identity matrix.

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Projection Transformations

• OpenGL supports two types of projections
  Orthographic and Perspective.
• Orthographic Projection
• For this type of projection the viewing volume is
  set up like a box.
• There is no real sense of depth in terms of
  visual cues (size or shape of object as it grows
  more distant).

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Projection Transformations

• Orthographic Projection

Two 3D Shapes
Appears flat and only depth cue is overlapping
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Projection Transformations

• Orthographic Projection
• This kind of projection are very useful when a 3D
  view complicates an area of interest.
• CAD and Modeling programs often make extensive
  use of orthographic projections.

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Projection Transformations

• Orthographic Projection
• To create the an orthographic projection matrix
  the function glOrtho is available.
• void glOrtho(GLdouble left, GLdouble right,
  
• GLdouble bottom, GLdouble
  top,
• GLdouble near, GLdouble far)

top
Note Even through the Z value into the screen
is increasingly negative in value the values
of near and far are set as positive values. But
they should never be equal to one another.
far
left
right
near
bottom
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Projection Transformations

• Perspective Projection
• This the projection model that represents how a
  camera (or eyes) view the world.
• As objects recede in the distance they grow
  smaller and we would see their sides.

Two 3D Shapes
Objects have perspective (in a PowerPoint
approximate kind of way)
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Projection Transformations

• Perspective Projection
• OpenGL provides two functions for automatically
  setting up a perspective projection.
• void glFrustum(GLdouble left, GLdouble right,
  
• GLdouble bottom,
  GLdouble top,
• GLdouble near, GLdouble
  far)
• void gluPerspective(GLdouble fovy,
• GLdouble aspect,
• GLdouble near,
• GLdouble far)

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Projection Transformations

• glFrustum is setup almost identically to glOrtho
  .
• void glFrustum(GLdouble left, GLdouble right,
  
• GLdouble bottom,
  GLdouble top,
• GLdouble near, GLdouble
  far)

By adjusting the size of the frustum, through any
of the parameters you can change the aspect
ratio of the scene to stretch it or squash it.
top
right
left
bottom
near
far
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Projection Transformations

• gluPerspective does the same thing a little more
  in straightforward way.
• void gluPerspective(GLdouble fovy,
• GLdouble aspect,
• GLdouble near,
• GLdouble far)

• fovy is the angle of the top and
• bottom planes of the frustum
• 0.0 to 180.0
• aspect is the aspect ratio which is
• the width divided by the height

w
h
fovy
near
far
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Projection Transformations

• gluPerspective actually calls glFrustum and here
  are how they are related.
• gluPerspective(fovy, aspect, near, far)
• height tan( fovY / 360 pi) near
• width height aspect
• glFrustum(-width, width, -height, height, near,
  far)

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Projection Transformations

• It is important to note that all these functions
  do is automatically construct a projection
  matrix. The matrix could also be constructed
  manually (and tediously) through calculations
  instead and applied directly.
• Also, the walls of the viewing volume also act as
  clipping planes. Anything inside the volume is
  drawn, anything outside is not, while anything on
  the border is automatically trimmed by OpenGL.

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Viewport

• A final optional part of resizeGL is setting the
  viewport which is the window which allows you to
  see the scene.
• By default the viewport is set to the size of the
  widget.
• However the viewport can be smaller and multiple
  viewports can be defined for multiple views in
  the same windows.

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glWidget paintGL

• This method is responsible for all your scene
  drawing and is called every time the timer fires.
• The general routine is to
• Clear the screen by overdrawing everything with
  the clear color
• Draw the scene
• Animation is achieved by changing the
  transformations of the objects to be drawn
  between screen clearings.

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glWidget paintGL

• Why erase everything every time? Why not just
  update the part that changes?
• It is easy to just redraw everything.
• Often it is also cheaper to redraw than to
  determine exactly what and how it needs to change
• Most dynamic applications (games) are going to
  routinely change most of the screen.

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Interacting with the Widget

• Most QWidgets have many methods which allow the
  user to interact with them. Since the GLWidget
  is a QGLWidget which is a QWidget then it
  inherits all those methods.
• Mouse, keyboard, touchpad with stylus,

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Mouse Events

• Every time the mouse moves it generates a
  QMouseEvent and passes it to mouseMoveEvent.
• Similarly when a mouse button is pressed it also
  generates a QMouseEvent and passes it to
  mousePressEvent.
• Contained within this Object are the x and y
  mouse positions, which buttons are currently
  pressed and if any modifier keys on the keyboard
  (Shift, Alt, Control) are pressed.

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Mouse Events

• Qt has several constants defined to represent
  each mouse button.
• None QtNoButton
• Left QtLeftButton
• Middle QtMidButton
• Right QtRightButton
• It also has representations for modifier keys.
• Shift QtShiftModifier
• Control QtControlModifier
• Alt QtAltModifer

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Mouse Events

• Getting the x and y coordinates within the
  widget.
• varX qme-gtx() varY qme-gty()
• Checking if a button is pressed
• if( qme-gtbutton() QtLeftButton )
• Checking if one of several buttons is pressed
• if( qme-gtbuttons() QtMidButton)

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Mouse Events

• Checking if a modifer key is pressed
• if( qme-gtmodifers() QtControlModifier )

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Keyboard Event

• Whenever a key is pressed on the keyboard it
  generates a QKeyEvent which is passed to the
  method keyPressEvent.
• Like with the mouse Qt has several constants
  defined to represent each key with a fairly
  predictable format.
• Escape QtKey_Escape
• Left Arrow QtKey_Left
• 5 QtKey_5
• T QtKey_T

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Keyboard Event

• Check if a key is pressed.
• if( kbe-gtkey() QtKey_H )
• Check if one of the modifiers is pressed.
• if( kbe-gtkey() QtShiftModifer )

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Keyboard Event

• Whenever a key is released on the keyboard it
  also generates a QKeyEvent which is passed to the
  method keyReleaseEvent which can be queried as in
  the previous examples.